Never use parentheses around multiply or divide, always use around add, subtract.
Use descriptionOfProgram: for history label.
Use updateDisplay method to update history.

Fix crash when using switchSign operation on negative operands.
Add buttons that push variables into the CalculatorBrain.
Use runProgram:usingVariableValues: instead of performOperation: to update display.

Create testVariableValues property within CalculatorViewController that contains NSString keys and NSNumber values.
Correct behaviour of switchSign operation when detecting closing parentheses.
Display values of variablesUsedInProgram: in label.
Extract updating of display with results to updateDisplay method.
Add "test" buttons to set testVariableValues to preset values.

Set x, y to 0 if testVariableValues is nil.
Clear testVariableValues with clear button.
Use separate methods to update history/variableValues and display.